Home > Articles > Mobile Application Development & Programming

This chapter is from the book

Using Protocols and Delegates

Many people consider protocols and delegates to be advanced topics in Objective-C. However, as you will see, they are critical in the table views that are often used to manipulate Core Data objects as well as in Core Data itself. They are also used throughout the iOS and Mac OS frameworks. This section explains that there are several pieces to the puzzle, but they fit together the same way in every case. Once you've worked through a few of them, they will become very natural. In particular, you will see that a lot of the details need no attention from you when you use a protocol or delegate. This section shows you how they work, but soon you will appreciate the fact that they are another part of the operating system that just works without too much of your attention.

Looking Up the Background of Protocols and Delegates

Object-oriented programming offered (and continues to offer) very powerful ways to build and maintain code. One issue arose quite early—multiple inheritance. For simple classroom examples, it is easy to propose a base class of Toy, with subclasses of Ball, Jump rope, and Puzzle. It is also easy to propose a base class of Sports Equipment with subclasses of Ball, Jump rope, Puzzle, Net, and Score board.

Both of these object hierarchies refer to real-life objects, and both make sense to most people. However, as soon as you start programming with those objects, you might find that you want an object such as a ball to have some variables and behaviors that descend from Toy and some that descend from Sports Equipment. In other words, can a ball have two superclasses (or ancestors)?

Many proposals have been made and implemented for solving the multiple inheritance problem. Objective-C started out addressing that issue and has evolved a structure that handles multiple inheritance. However, it also covers a number of other long-time object-oriented programming issues.

The Objective-C approach that has evolved allows you to share functionality between two objects without using inheritance. To be sure, inheritance is used throughout Objective-C, but the very deep inheritance hierarchies that often evolve in languages such as C++ are far less common in Objective-C. Instead, you can take a defined chunk of functionality and share it directly.

A major distinction between extending a class by subclassing it and extending a class by adding a protocol to it is that a subclass can add or modify methods and can also add new instance variables. Protocols, like categories that are described briefly at the end of this section, only add methods.

Using an Example of a Protocol

An example of this is found in the iOS sample code for Multiple Detail Views (http://developer.apple.com/library/ios/#samplecode/MultipleDetailViews/). This code addresses an issue that arises with some iPad apps. iOS supports a split view in which the main view fills the screen when the device is vertically oriented; when the device is horizontally oriented, the right and larger part of the screen shows the detail view, but, at the left, a list of items controls what is shown in the larger view.

In the vertical orientation, a control bar at the top of the window contains a button that will let you open a popover with the list of items that can be shown at the left.

The problem arises because the control bar at the top of the window can be a navigation bar or a toolbar. These are two different types of controls. The button to bring up the popover needs to be shown (in portrait mode) and hidden (in landscape mode). The code to implement this differs whether the button is added to a toolbar or to a navigation bar.

The key to this consists of four steps:

  • Declaring a protocol—A protocol is declared. It is a set of methods presented as they would be in an interface.
  • Adopting the protocol—Any class in this sample app that wants to be able to use this protocol must adopt it in its header. Adopting a protocol means that the class declared in the header must implement methods from the protocol. (Note that ones marked optional do not have to be implemented. This is another Objective-C 2.0 improvement.)
  • Implementing the protocol—Any class that adopts the protocol must implement all required methods and might implement other methods. The implementations might use variables and other methods of the particular class that adopts the protocol.
  • Using the protocol.

The code is described in the following sections.

The first step is to define the protocol in RootViewController.h, as shown in Listing 3.6.

Listing 3.6 Defining the Protocol

@protocol SubstitutableDetailViewController
- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;
- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;

Beginning with Objective-C 2.0, you can indicate which methods are required or optional. The default is required, so the code in Listing 3.6 actually is the same as the code shown in Listing 3.7.

Listing 3.7 Marking Protocol Methods Required or Optional

@protocol SubstitutableDetailViewController
- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;
- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;

After you have declared a protocol, you need to adopt it. Listing 3.8 shows the code from the sample app for a view with a toolbar. Although the protocol is declared in RootViewController.h, it is adopted in FirstDetailViewController.h (and in the second one, too).

Listing 3.8 Protocol Adoption with a Toolbar

@interface FirstDetailViewController : UIViewController <
SubstitutableDetailViewController> {

  UIToolbar *toolbar;

Listing 3.9 shows the protocol adopted by another view that uses a navigation bar.

Listing 3.9 Protocol Adoption with a Navigation Bar

@interface SecondDetailViewController : UIViewController <
SubstitutableDetailViewController> {

  UINavigationBar *navigationBar;

Each of the classes that has adopted the protocol must implement its methods. Listing 3.10 shows the implementation of the protocol with a toolbar in FirstDetailViewController.m.

Listing 3.10 Implementation of the Protocol with a Toolbar

#pragma mark -
#pragma mark Managing the popover

- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem {

// Add the popover button to the toolbar.

NSMutableArray *itemsArray = [toolbar.items mutableCopy];
  [itemsArray insertObject:barButtonItem atIndex:0];
  [toolbar setItems:itemsArray animated:NO];
  [itemsArray release];

- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem {

// Remove the popover button from the toolbar.

NSMutableArray *itemsArray = [toolbar.items mutableCopy];
  [itemsArray removeObject:barButtonItem];
  [toolbar setItems:itemsArray animated:NO];
  [itemsArray release];

In Listing 3.11, you see how you can implement the protocol with a navigation bar. (This code is from SecondDetailViewController.m.)

Listing 3.11 Implementation of the Protocol with a Navigation Bar

#pragma mark -
#pragma mark Managing the popover

- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem {

// Add the popover button to the left navigation item.
  [navigationBar.topItem setLeftBarButtonItem:barButtonItem animated:NO];

- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem {

// Remove the popover button.
  [navigationBar.topItem setLeftBarButtonItem:nil animated:NO];

The next step is to adopt another protocol. This protocol, UISplitViewControllerDelegate, is part of the Cocoa framework, so you do not have to write it. All you have to do is adopt it as the RootViewController class in the example does. The Interface is shown in Listing 3.12 together with the adoption of the protocol in RootViewController.h. To repeat, what that adoption statement (in the < and >) means is that all required methods of the protocol will be implemented by this class.

Listing 3.12 Adopting the UISplitViewControllerDelegate Protocol

@interface RootViewController : UITableViewController
  <UISplitViewControllerDelegate> {
UISplitViewController *splitViewController;

UIPopoverController *popoverController;
UIBarButtonItem *rootPopoverButtonItem;

Having promised to implement the required and (possibly) optional methods of the UISplitViewControllerDelegate protocol, RootViewController.m must do so. The sample app implements two of the methods as shown in Listing 3.13. In doing so, it has fulfilled the promise made when it adopted the UISplitViewControllerDelegate protocol.

There are two critical lines, one in each method of Listing 3.13. Those lines are the same in both methods and are underlined. It is easiest to start reading them from the middle. The heart of each line is the assignment of a local variable, *detailViewController, using the split view controller's array of view controllers and selecting item one.

This local variable is declared as being of type UIViewController and adopting the SubstitutableDetailViewController protocol shown previously in Listing 3.6. Because it adopts the protocol, it is safe to assume that it implements all the required methods. Because nothing is marked optional, both methods are required, so it is certain that they will be there (if they are not, that assignment statement will fail).

Listing 3.13 Implementing the protocol in RootViewController.m

- (void)splitViewController:(UISplitViewController*)svc
  willHideViewController:(UIViewController *)aViewController
  forPopoverController:(UIPopoverController*)pc {

// Keep references to the popover controller and the popover button, and tell the
// detail view controller to show the button.
  barButtonItem.title = @"Root View Controller";

self.popoverController = pc;

self.rootPopoverButtonItem = barButtonItem;

UIViewController <SubstitutableDetailViewController> *detailViewController =
   [splitViewController.viewControllers objectAtIndex:1];
   [detailViewController showRootPopoverButtonItem:rootPopoverButtonItem];

- (void)splitViewController:(UISplitViewController*)svc
  willShowViewController:(UIViewController *)aViewController
  invalidatingBarButtonItem:(UIBarButtonItem *)barButtonItem {

// Nil out references to the popover controller and the popover button, and tell
// the detail view controller to hide the button.

UIViewController <SubstitutableDetailViewController> *detailViewController =
   [splitViewController.viewControllers objectAtIndex:1];
   [detailViewController invalidateRootPopoverButtonItem:rootPopover

self.popoverController = nil;

self.rootPopoverButtonItem = nil;

You might have to trace through the code again, but it is worth it to get the hang of it. The point is that this locally declared class inherits from a standard class in the framework (UIViewController). However, by creating and adopting its own protocol, two separate classes with two different ways of implementing control bars can both promise to do the same thing, albeit in different ways because they have different types of control bars to work with.

Using Delegates

Protocols are often paired with delegates, another key Objective-C concept. As noted previously in this hour, instead of calling procedures, messages are sent to objects in Objective-C. That makes the use of delegates possible. A class can declare a delegate for itself. That delegate processes messages sent to the object itself. Frequently, functionality is wrapped up in a protocol as you have seen here, and some of those protocols are designed to be used by delegates.

For example, you saw in Listing 3.12 that the RootViewController class adopts the UISplitViewControllerDelegate protocol. This means that a RootViewController can be named as the delegate of an object that requires that protocol to be implemented.

  • This is a high-level view of delegates and protocols. You will find more examples and much more detail in Part IV, "Using Data Sources and Table Views." If it is a little fuzzy now, do not worry.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020